In recent years, the use of advanced composite structures has experienced tremendous growth in the aerospace, automotive, and many other commercial industries. While composite materials offer significant improvements in performance, they require strict quality control procedures in both the manufacturing processes and after the materials are in service in finished products. Specifically, non-destructive evaluation (NDE) methods must assess the structural integrity of composite materials. This assessment detects inclusions, delaminations and porosities. Conventional NDE methods are slow, labor-intensive, and costly. As a result, testing procedures adversely increase the manufacturing costs associated with composite structures.
Various methods and apparatuses have been proposed to assess the structural integrity of composite structures. One solution uses an ultrasonic source to generate ultrasonic surface displacements in a work piece which are then measured and analyzed. Often, the external source of ultrasound is a pulsed generation laser beam directed at the target. Laser light from a separate detection laser is scattered by the surface the work piece. The detection laser light is phase modulated by the ultrasonic displacements. Notice that a modulation of phase as a function of time corresponds also to a frequency modulation and either type of modulation can be used to describe the process depicted here. Collection optics then collect the scattered laser energy. The collection optics are coupled to an interferometer or other device. The interferometer demodulates the ultrasonic displacement informant and data about the structural integrity of the composite structure can be obtained through analysis of the resulting signal. Laser ultrasound has been shown to be very effective for the inspection of parts during the manufacturing process.
However, the equipment used for laser ultrasound is custom-designed and is presently a limiting factors regarding inspection speed. Previous solid-state detection lasers used either flash-lamp pumped rod architectures or diode-pumped slab configurations to amplify a low power master oscillator laser. These configurations are generically referred to as master oscillator power amplifier (MOPA) lasers.
Inspection speed is currently limited by the pulse rate of the lasers. Flash-lamp pumped lasers can only operate at 100 Hz and the lamps typically only last 10's of millions of shots. Therefore these lasers are slow and expensive to operate. Diode-pumped slabs are much faster (400 Hz is current limit and 1 Khz may be possible) but they use very expensive custom-manufactured diode arrays to pulse-pump the slabs and create a great amount of heat which can induce thermal distortion. Although diode array lifetimes are getting better, some have lasted 102 shots, they have historically been a concern due to both high-cost, reliability and thermal distortion. High-power pulsed-diode pumping of a crystal slab will introduce thermal distortions into the slab that ultimately limits the waveform quality of the laser beam. Wavefront distortion can limit the useful power of a laser and prevent efficient fiber optic delivery of the beam to the target. Each diode bar in the array may have a peak power of 40 W to 100 W and they must be physically close to each other in order to efficiently pump the side of the laser slab. The total number of diode bars in an array may be 50-100 (an array will pump each side of the slab, so possibly 200 diode bars may be used). Heat removal is a significant design issue for both the diode arrays and the slab.